Chinese jet fighters have been sent to the South China Sea, the People’s Liberation Army Air Force announced on Wednesday in a move that a retired major general said could be a reaction to provocation by the US in January.

The Su-35 Russian-made advanced fighters were dispatched to the sea for joint combat cruise missions, according to the official Sina Weibo account of the PLA Air Force. The message included photos but did not indicate how many aircraft.

This is a pragmatic action for the air force to fulfil its mission in the new era and conduct a combat training military exercise,” according to a notice on the PLA Air Force official Weibo social media account.

The Su-35 is a multirole fighter aircraft that can attack targets on the ground and the sea. The fighter can significantly improve the combat capability of the air force overseas.

“Apart from the most advanced J-20, a China-made fourth generation stealth fighter jet, the Su-35 is more advanced than other Chinese fighter jets at this moment,” Xu Guangyu, a retired major general and senior adviser to the China Arms Control and Disarmament Association, told the Global Times on Wednesday.

The deployment showed that pilot training for new jets was surprisingly fast, Xu noted.

“We just received a group of jets from Russia last year, and now we can put them into a real combat mission in the South China Sea,” Xu said.

“It also shows that China-Russia military cooperation is solid, mutual-benefited and reliable.”

China reportedly bought 24 Su-35 fighter jets for $2 billion from Russia in 2015, and Russia delivered them all by the end of 2017.

The US sent destroyers to provoke China in the South China Sea last month, Xu said.

“The appearance of advanced PLA fighter jets, capable of attacking surface combat vessels in this region is sort of a reaction to the provocation by the US,” Xu said.

Although the air force didn’t reveal where the jets are deployed, if they land on a South China Sea island that could act as a significant deterrent to any outside forces trying to disturb the regional stability, he said.